You will not find a requirement in a guidance on stability studies but rather the requirement came from a 483 observation. Thus, an HPLC validation chemist normally performs a comparison of concentration results between a freshly prepared working standard (derived from the stock solution), sample, and mobile phase for ~5 days.

This is usually done during method validation, and I think the reason behind that is to prove that the working solution is stable during analysis and not degraded in the diluent media, thus obtaining incorrect results, especially if you have large number of samples to be analyzed and therefore a long time between samples, otherwise you should mention in your method that analytical solutions should be freshly prepared.

Studying the stability of a stock solution is indeed crucial, especially if you plan to use it over an extended period or if you have a large number of samples to prepare or analyze. Here are some reasons why it's important:

Ensures Accurate and Reproducible Results: The stability of a stock solution can affect the accuracy and reproducibility of your experiments or analyses. If the concentration of the solute in the stock solution changes over time, it can lead to inconsistent results in your experiments, making it difficult to draw meaningful conclusions.

Cost-Efficiency: Stock solutions are often prepared in larger quantities to save time and resources. If the stock solution degrades or becomes less stable over time, you may need to prepare new solutions more frequently, which can be costly in terms of reagents and labor.

Saves Time: If you have a large number of samples to process, having a stable stock solution eliminates the need to prepare fresh solutions each time, saving you valuable time in your research or analytical work.

Quality Control: Monitoring the stability of a stock solution is a part of quality control in many laboratories. It ensures that the solutions used in experiments or analyses meet the necessary standards and specifications.

Long-Term Experiments: Some experiments or research projects may span several months or even years. To maintain the integrity of your work over such extended periods, it's essential to have confidence in the stability of your stock solutions.

To study the stability of a stock solution, you typically perform stability testing under controlled conditions. This may involve monitoring the concentration of the solute over time, checking for any physical changes in the solution (such as precipitation or color changes), and evaluating the solution's performance in your specific applications.

Stability studies often include storing the stock solution under different conditions, such as varying temperatures and light exposure, to assess its stability under different environmental factors. The results of these studies can guide you in determining the appropriate storage conditions and shelf life of your stock solution, helping you make informed decisions about its use in your research or analyses.